Broadband access technology has lagged the consumer demand for many years. During 20th century, a broadband backbone networks on primarily fiber optical has been built across the globe during the dot-com boom. However, the so-called “last mile” problem has prevented millions of homes from reaching the backbone networks and benefiting from truly broadband applications with video, voice, and data. It is certain that the future of the broadband access lies in the delivery of triple-play services of video, voice and data, which will eventually enable tremendous business and consumer opportunities such as real-time video phone, video-on-demand, video conference, virtual reality online shopping, etc.
Basically there are four competing access technologies: fiber optical, wireless, cable, DSL. EPON and GPON are two primarily fiber optical technologies. EPON stands for Ethernet Passive Optical Networks and GPON stands for Gigabit-capable Passive Optical Networks. EPON and GPON can deliver the highest data rate among all the technologies. At the same time, it has the highest initialization, operation and maintenance cost. So, the major problem with fiber optical is purely economical. Cable modem uses shared cable medium and split the total bandwidth among all connected customers. This brings the security concern and scalability problem when the number of end users starts to increase. WIMAX is a promising wireless technology for broadband access. Due to the nature of wireless, the performance may not be very reliable especially for longer distance and no line-of-sight. However, it caters nicely to the mobile applications and certainly will capture certain market share in the broadband access. DSL technology led by VDSL2, we believe, is the key enabling technology that bridging the access between the fiber optical backbone networks and the end customers. It will eventually bring economical broadband access to millions of home, which will in turn once again energize Internet through triple-play applications.
VDSL2 refers to second-generation very-high speed digital subscriber line and the first draft standard (G.993.2) was proposed in May 2005 by the International Telecommunication Union (ITU). VDSL2 is an evolving DSL technology that aiming at delivering high data rate through copper pairs. The supported data rate can be up to 100 Mbps at each direction of downstream and upstream. Different profiles are created in order to meet the requirement of different deployment scenarios mostly related to the loop distance. The following table 1 shows all the supported profiles:
TABLE 1G.993.2 - VDSL2 profilesFre-quen-cyParameter value for profileplanParameter8a8b8c8d12a12b17a30aAllMaximum+17.5+20.5+11.5+14.5+14.5+14.5+14.5+14.5aggregatedownstreamtransmitpower (dBm)AllMaximum+14.5+14.5+14.5+14.5+14.5+14.5+14.5+14.5aggregateupstreamtransmitpower (dBm)AllSub-carrier4.31254.31254.31254.31254.31254.31254.31258.625spacing(s)(kHz)AllSupport ofRequiredRequiredRequiredRequiredRequiredNotNotNotupstreamRequiredRequiredRequiredband zero(US0)AllMinimum net50 Mbit/s50 Mbit/s50 Mbit/s50 Mbit/s68 Mbit/s68 Mbit/s100 Mbit/s200 Mbit/saggregate datarate capability(Mbit/s)AllAggregate65,53665,53665,53665,53665,53665,53698,304131,072interleaverand de-interleaverdelay (octets)AllDmax20482048204820482048204830724096All1/Smax2424242424244828downstreamAll1/Smax1212121224242428upstream
Lower profiles such as 8a˜12b are used to support medium range loop length with a distance between 3 kft to 8 kft while high speed profiles 17a˜30a are used to support short range loop length of less than 3 kft. Only 30 MHz profile 30a is able to support 100 Mbps on both upstream and downstream while 17 MHz profile 17a can support aggregated 100 Mbps.
Discrete-Multi-Tone is used as the basic modulation scheme for VDSL2. The total bandwidth is split into multiple smaller sub-carriers through IFFT and FFT engines. All profiles except 30 MHz has sub-carrier tone spacing of 4.3125 KHz, while 30 MHz profile has sub-carrier tone spacing of 8.625 KHz in order to support a total bandwidth up to 30 MHz with 4096 sub-carriers.
FFT and IFFT engines are two major blocks in DMT-based VDSL2 communication system. Due to the fact that the FFT and IFFT modules use significant number of multipliers and adders as well as memory, they incur significantly larger die cost than other modules in VDSL2 system. On the other hand, the FFT and IFFT modules need to run at very high rate in order to achieve the system frame rate and sampling rate requirement, they consume a large percentage of the total digital power. This is especially true for high data rate based VDSL2 profiles such as 30 MHz or 17 MHz.
In order to improve the system performance, DMT-based VDSL2 systems standardize the prefix and suffix insertion to reduce ISI and the transmitter shaping to the frame boundary. In addition, the receiver windowing is employed at the receiver side to further reduce the effect of crosstalk and narrow-band interference. However, the inclusion of the above methods requires a re-ordering of the transmitter and receiver time-domain samples, which usually requires a large time-domain buffering memory.
In term of architecture design, 30 MHz profile imposes biggest challenge because of its highest sampling rate (at least 69 MHz Nyquist rate). It requires very careful design on FFT and IFFT to meet the timing requirement based on different processing technologies. On the other hand, if multiple profiles are supported, the FFT/IFFT design architecture needs to efficiently accommodate different FFT/IFFT sizes. If all VDSL2 profiles are supported and also the design is backward capable with ADSL2+, the following FFT/IFFT sizes are needed (without over-sampling): 8192 (for 30a, 17a, 12a, 12b), 4096 (for 8a, 8b, 8c, 8d), 1024 (for ADSL2+). Our FFT/IFFT design is capable of supporting all FFT/IFFT sizes of 2i, i can be any positive integer while it is flexible enough to choose the supported sizes to reduce the hardware cost. In the case of VDSL2, we can configure it to support up to 8192 while limiting the choices of allowed sizes to be only 2i, i=6, 10, 12, 13. It is apparent that this configuration is the minimum set to support all required profiles and ADSL2+ compatibility and has the minimum hardware cost.
On the transmitter side, the IFFT engine converts frequency-domain tones to time-domain samples. Each frequency-domain tone is modulated by QAM (Quadrature Amplitude Modulation) signal. Each QAM modulation can carry up to 15 information bits, which can be un-coded or coded depending on if or not TCM (Trellis-Code-Modulation) is used. Due to the communication channel dispersion, the cyclic prefix is added at the beginning of the transmitter frame so that the previous DMT frame does not interference with the current DMT frame. Of course, the cyclic prefix will be affected by the previous DMT frame and therefore it is removed from the DMT frame on the receiver side. For longer loop, usually a Time-Domain Equalizer (TEQ) is employed to shorten the channel so that the prefix length can be reduced.
In addition, in order to maintain orthogonality between the disturbing transmit signal and the received signal, the cyclic suffix is added to the transmitter DMT frame to protect the frame from the self echo and near-end crosstalk (NEXT). The cyclic suffix is used together with a timing advance mechanism to make sure that the transmitters and receivers are sufficiently aligned in time. If the time-advance is properly set, the orthogonality can always be maintained as far as the total length of the cyclic prefix and cyclic suffix is longer than the time-dispersion and propagation delay of the communication channel.
On the receiver side, the prefix and suffix segments are stripped from the received DMT frame while the received samples protected by the prefix and suffix are sent to the FFT engine. The FFT engine then converts the time-domain samples back to the frequency-domain tones and the information bits are extracted. In order to further mitigate the effect of near-end crosstalk and narrow-band crosstalk, usually the receiver windowing is used to smooth the received protected frame on the boundary with the prefix and suffix segments.
The prefix and suffix insertion and the receiver windowing complicate the time-domain data sequence. On the transmitter side, it requires memory buffers to store the time-domain data from the IFFT. Then the prefix and suffix are inserted based on buffered time-domain data. While on the receiver side, the time-domain data has to be buffered first before a normal FFT frame is extracted from the buffered data and sent to the FFT engine.